Synaptic and Neuromuscular Transmission PDF

Week 2: Synaptic and Neuromuscular Transmission Differentiating Electrical and Chemical Synapses Synaptic and Neuromuscular Transmission A synapse is a site where information is transmitted from one cell to another. The information can be transmitted either electrically (electrical synapse) or via...

Week 2: Synaptic and Neuromuscular Transmission Differentiating Electrical and Chemical Synapses Synaptic and Neuromuscular Transmission A synapse is a site where information is transmitted from one cell to another. The information can be transmitted either electrically (electrical synapse) or via a chemical transmitter (chemical synapse). Types of Synapses Electrical Synapses Electrical synapses allow current to flow from one excitable cell to the next via low-resistance pathways between the cells called gap junctions. Gap junctions are found in cardiac muscle and in some types of smooth muscle and account for the very fast conduction in these tissues. For example, rapid cell-to-cell conduction occurs in cardiac ventricular muscle, in the uterus, and in the bladder, allowing cells in these tissues to be activated simultaneously and ensuring that contraction occurs in a coordinated manner. Chemical Synapses In chemical synapses, there is a gap between the presynaptic cell membrane and the postsynaptic cell membrane, known as the synaptic cleft. Information is transmitted across the synaptic cleft via a neurotransmitter, a substance that is released from the presynaptic terminal and binds to receptors on the postsynaptic terminal. The following sequence of events occurs at chemical synapses: An action potential in the presynaptic cell causes Ca²⁺ channels to open. An influx of Ca²⁺ into the presynaptic terminal causes the neurotransmitter, which is stored in synaptic vesicles, to be released by exocytosis. The neurotransmitter diffuses across the synaptic cleft, binds to receptors on the postsynaptic membrane, and produces a change in membrane potential on the postsynaptic cell. The change in membrane potential on the postsynaptic cell membrane can be either excitatory or inhibitory, depending on the nature of the neurotransmitter released from the presynaptic nerve terminal. If the neurotransmitter is excitatory, it causes depolarization of the postsynaptic cell; if the neurotransmitter is inhibitory, it causes hyperpolarization of the postsynaptic cell. In contrast to electrical synapses, neurotransmission across chemical synapses is unidirectional (from presynaptic cell to postsynaptic cell). The synaptic delay is the time required for the multiple steps in chemical neurotransmission to occur.